A semiconductor model of the passive layer on iron electrodes and its application to electrochemical reactions

The semiconductor model of the passive layer recently derived from capacity measurements is used for the explanation of transfer reactions of electrons (etr) and ions (itr) on passive iron electrodes. Etr takes place via the conduction band at low potentials and via the valence band at high potentials. Current potential curves of etr can be explained taking into account the semiconducting properties of the film and tunnel probabilities. Itr of ferrous and ferric ions are determined by the potential drop in the Helmholtz layer which is constant in the passive region but increases in the transpassive region because of an increasing hole concentration in the valence band. Film reduction and anodic deposition of γ-FeOOH-layers are coupled reactions which depend on carrier concentrations and tunnel probabilities as well as on the potential drop in the Helmholtz layer. The proposed semiconductor model is consistent with the structure of the passive film as well as the known kinetics of reactions occurring on passive iron electrodes.